1. Field of the Invention
This invention relates to an optical phosphate glass in which the temperature dependence of the length of the light path is nearly zero and which has a small Abbe number.
2. Description of the Prior Art
It is well known that when a temperature gradient occurs in glass, the length of the light path changes, and the wave front is distorted in the interior of the glass. This reduces the ability of the glass to form a focused image. For example, when a glass is used in a laser system, a temperature gradient is developed inside the glass by localized heating induced by the laser beam, and consequently, the beam is distorted. When glass is exposed to extreme temperature changes, for example, optical instruments set in a spacecraft, the length of the light path changes due to the temperature gradient generated in the glass, and a distortion occurs in the wave front, which in turn results in a reduction in the ability of the glass to form a focused image. Therefore, it would be very advantageous to construct an optical system using a glass in which the length of the light path is quite independent of temperature.
Changes in the length of the light path based on temperature changes in a glass sheet whose optical surfaces are parallel planes can be calculated using the following equation. EQU .increment. W.sub.G = [.alpha.(n-1) + dn/dT].multidot.l.multidot..increment. T
or if EQU .increment. W.sub.G = G .multidot. l .multidot. .increment. T
then, EQU G = .alpha.(n-1) + dn/dT
wherein:
.alpha. IS THE COEFFICIENT OF THERMAL EXPANSION OF THE GLASS (IN .degree. C.sup.-1);
n is the refractive index of the glass;
L IS THE THICKNESS OF THE GLASS SHEET, AND
.increment.T represents the temperature change (in .degree. C).
When .increment. w.sub.G changes due to the occurrence of a temperature gradient in various parts of the glass sheet then a distortion occurs in the wave front. In the above equation, the term EQU G = .alpha.(n-1) + dn/dT
is the part of the equation at which the change of the length of the light path is determined by the composition of the glass sheet. Hence, by reducing G to 0, the change in the length of the light path based on the temperature gradient in the glass can be removed. For this purpose, dn/dT should be a negative value since .alpha.(n-1) normally has a positive value.
Separately, there is a change in the length of the light path which is caused by thermal stress in the glass. This change is shown by the following equation. EQU .increment. W.sub.s = .differential.n/.differential..sigma..multidot.d.sigma./dT.multidot.l.mult idot..increment.T
wherein .sigma. represents the thermal stress, n is the refractive index of the glass, l is the thickness of the glass and .increment. T is the temperature change.
Since .increment.W.sub.s normally has a positive value, in order to reduce the change of the total length of the light path to zero, i.e., EQU .increment. W = .increment.W.sub.G + .increment.W.sub.s = 0
.increment. W.sub.G must be &lt;0, and hence, G &lt; 0.
Glasses having a small Abbe number and where the length of the light path is independent of temperature must be rendered achromatic by combining glasses having a large Abbe number.
Japanese Patent Application (OPI) No. 37109/76 discloses a glass having an Abbe number .nu.d in the range of 30 to 45 and where the length of the light path is nearly independent of temperature. This glass, however, is a fluoroborosilicate glass, and poses many manufacturing problems because the glass contains fluorine.
Japanese Patent Publication 1091/73 and Japanese Patent Application (OPI) No. 37108/76 also disclose a glass which is free from a temperature dependence. The glass disclosed in Japanese Patent Publication 1091/73 has a refractive index of 1.5 or less and an Abbe number of 50 to 65 and contains fluorine which involves many manufacturing problems as in Japanese Patent Application (OPI) No. 37109/76. Further, the glass disclosed in Japanese Patent Application (OPI) No. 37108/76 has a refractive index of 1.59 to 1.65 and an Abbe number of 55 to 46, which are narrower than those of this invention.